In this study, we investigated the relationship between the blood glucose level and the length of the QT-interval as well as the influence of locomotor activity on these characteristics in children and adolescents presenting with type 1 diabetes mellitus during the long-term monitoring of ECG, the level of glycemia, and locomotor activity. The daily dynamics of these parameters was elucidated along with the measurements of the lengths of QT and RR intervals. Specifically, the correlation between the length of the QTc interval and the blood glucose level appears from the fact that the maximum QTc length occurred when glycemia dropped below 4 mmol/l. It also significantly increased at glycemia in excess of 16 mmol/l (and up to 21 mmol/l). Moreover the length of the QTc interval directly depended on the duration of the disease. The locomotor activity was shown to significantly affect the blood glucose level within 3-4 hours after its onset.